An inverted duplication of chromosome 15 (inv dup[15] chromosome) is the most common supernumerary marker chromosome in humans. Inv dup(15) chromosomes are commonly associated with mental retardation, epilepsy, behavioral problems and structural malformations. Though epilepsies associated with inv dup(15) chromosomes are often intractable, there have been very few reports regarding the seizure manifestations or types. We report a patient with severe mental retardation and intractable epilepsy, associated with an inv dup(15) chromosome. The seizures recorded with EEG-VTR monitoring were axial and generalized tonic seizures, and our case was diagnosed as symptomatic generalized epilepsy. Molecular and cytogenetic analysis showed an inv dup(15) chromosome containing the Prader-Willi syndrome/Angelman syndrome region mapped within bands 15q 11-q13.